It's late afternoon in a terraced house in Hull, and the Clark brothers are battling it out at Monopoly at the dining room table. "They love board games – they'd play them all day," says their mother, Christine. "The only problem," adds Tony, their father, "is when one of them finds out he hasn't won, Then you end up with a huge fight, with the board and the pieces on the floor."

It sounds like the sort of thing that could happen in any home with a couple of primary school-aged kids. But the terrible reality is that the children in question are 42 and 39. An extremely rare medical condition means Michael and Matthew Clark aren't in their first childhood, they're going through their second – and for their parents, the agony is almost unendurable.

The Clarks don't like the boys hearing the story – and they'd never sit still for long enough to allow us to talk – so we retreat to the sitting room while they continue their game with the carers who come in each afternoon to give Tony and Christine a break. On the mantelpiece, there's a picture of the boys aged around 13 and 10. But the reality, says Christine, 62, is that in many ways they were more mature then than they are now.

Looking back, Tony and Christine can see there were warning signs, through the years, that all was not well – but the situation they were warning about was so bizarre and shocking that no one could possibly have heeded them. "I remember taking the boys to Spain for a holiday," says Tony, 63. "They were both in their thirties, and I remember, as the plane took off, they were shouting out 'Yippee' like a pair of kids."

On the same holiday, Christine remembers having a nagging sense that something wasn't right. "The boys kept squabbling, and they seemed really childish. I thought it was odd; by that stage Matthew was a dad himself."

Both Michael and Matthew had done well at school. Matthew was offered places both in the Royal Navy and at agricultural college, though he ended up doing a variety of training schemes. Michael went into the RAF at the age of 20, and later qualified as a cabinet maker. Both men married: Matthew and his wife had a daughter, Lydia – now 19 – and Michael had stepchildren. "They left home, settled down, and all seemed to be going well," says Christine.

By this stage, Tony was in his late forties. He decided to take early retirement from his job as a prison officer. "And then we decided to move to Spain," says Christine. So seven years ago, believing their years of heavy-duty childrearing were well behind them, they sold their home in Gloucestershire and moved to a village near Benidorm.

The plan was for Michael and Matthew to visit whenever they could, but from the outset things seemed strange. "They never answered their mobiles – I kept calling and calling," says Tony. "And then, one day, Lydia phoned to say a worker from a hostel had called to say her dad was living there, and there were some problems he needed to discuss."

Tragically, Tony and Christine's departure had coincided with their sons' downward spiral. "If we'd been in the UK, we'd have realised something was very wrong," says Christine. Instead, Michael and Matthew, who were by this stage both divorced, ended up on benefits, sharing an increasingly squalid flat in Lincoln – and they became gradually unable to look after themselves. By the time their parents came back, the two were like a pair of toddlers unable to cope in an adult world. "The flat was an absolute tip, with the washing all piled up in one corner, and the kitchen took me hours and hours to clean up," says Christine. "It was unbelievable."

By this stage, the brothers had been squabbling so much that Michael had moved out of the flat and into a hostel. Workers there arranged for him to have medical checks: and when doctors realised he had a brother with similar problems, they ran a series of genetic tests. The results were devastating: both brothers were found to have terminal leukodystrophy. The boys' brains were being destroyed: intellectually and emotionally, they were returning to their babyhood.

For Tony and Christine, there was only one possible course of action: earlier this year they left Spain for good (though because of the economic situation, they've been unable to sell their house) and came back to the UK to look after their children full-time. This summer, Hull Council gave them a specially adapted house, and they're now living a back-to-the future existence as parents of two children who are, to all intents and purposes, getting younger by the day.

In almost every way, life for Tony and Christine is as it was three decades ago – the only difference being that the "toddlers" they're caring for are six-feet-tall adults. "Michael is the most child-like: he's moody, he can't be left on his own. Matthew talks all the time, he says whatever comes into his head, and he tends to make big noises and to shout out a lot," says Christine.

"They can be very affectionate, particularly with one another – they'll often put their arms around one another, and Michael will say, 'He's my little brother'. Just like small children, they wake up a lot during the night – I was up seven times with them last night – and, also like children, they'll deny and deny that they're tired, even when you can see their eyelids drooping."

At the moment, both boys eat normally but, says Christine, it's only going to be a matter of time before that will be too difficult for them. "A few weeks ago, they could still manage with a knife and fork, but now that's getting too difficult for them – they get the food on to their forks, but somehow it all falls off before it reaches their mouths."

Walking, too, is becoming increasingly difficult: Matthew uses a wheelchair when he's out, and Michael is finding walking more and more of a struggle. "I took him to the supermarket the other day, and I felt this big hand reaching out for mine," says Tony sadly. "So we walked around hand in hand: we got a few stares, but if it's what he needs to do, it's what we're doing."

The boys have been called the "Benjamin Button brothers", after the F Scott Fitzgerald character who is born as an old man and grows up to be a child. But Christine says the analogy is upsetting. "For one thing, they're not getting smaller – there's no return to them being cute little boys, they're big strong men – and that presents a quite different set of problems," she says.

Most tragically of all, there are occasions when her sons – especially Michael – are all too aware of their situation. "There are times when they know what they had, they know they were once normal adults with normal lives," says Christine. "They're aware of what it is they're losing." She and Tony, she says, are aware that both their sons might die before they do. "My biggest fear is that they'll outlive us, because it would be terrible to think of them having to survive without us," she says. "But I don't know how we'll begin to deal with losing them."

One of the few things that make them happy, these days, is being around children – at a family event in the summer, says Christine, they enjoyed splashing around in the paddling pool and playing with a water gun. They don't get much chance to mix with children – although, in a sad twist of fate, Matthew recently became a grandfather when Lydia gave birth to a son called Zachery. "She brought him over to meet us, but it was very hard all round," says Christine. "Matthew loved him, and he knows he's his grandson – but in some ways he seemed more like a slightly older brother. It all feels so wrong – this is so absolutely not how anyone's life should ever be."

'The Curious Case of the Clark Brothers' is broadcast tomorrow at 9pm on Channel 4

What is leukodystrophy?

* Leukodystrophy is a neurological disease which affects the brain, nervous system and spinal cord.

* It attacks the white ("leuko") matter of the spinal cord and brain, causing gradual loss of brain functions.

* It cannot be cured, and there is no medical treatment to slow the symptoms.

* It's extremely rare – there are only around 100 people affected in the UK, most of them children.

* There are around 40 known types, and research is ongoing into the cause and possible treatments.

* Most forms of the condition are inherited from parent carriers of the gene deficiency. However, there's only a one in three billion chance of two carriers meeting and deciding to have children together.

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